Toners which melt at lower temperatures have become an industry standard. Conventional electrophotographic processes require temperatures of 160.degree.-200.degree. C. to fix toner on a support medium, e.g., a sheet of paper or transparency, creating a developed image. Such high temperatures may reduce or minimize fuser roll life, such as fuser rolls made of silicone rubbers or fluoroelastomers (e.g., Viton.RTM.), limit fixing speeds, or necessitate higher power usage during operation, such as in a xerographic copier employing a hot roll fixing mechanism.
Electrophotographic toners are generally prepared by mixing or dispersing a colorant and possibly a charge enhancing additive into a thermoplastic binder resin, followed by micropulverization. Known, conventional thermoplastic binder resins include polystyrenes, styreneacrylic resins, styrene-methacrylic resins, polyesters, epoxy resins, acrylics, urethanes and copolymers thereof. Carbon black is often used as a colorant and alkyl pyridinium halides, distearyl dimethyl ammonium methyl sulfate, and the like are employed as charge enhancing additives.
Although many processes exist for fixing toner to a support medium, hot roll fixing, transferring heat very efficiently, is especially suited for high speed electrophotographic processes. In this method, a support medium carrying a toner image is transported between a heated fuser roll and a pressure roll, the image face contacting the heated fuser roll. Upon contact with the fuser roll, the toner melts and adheres to the support medium, permanently fixing an image.
Toner fixing performance may be characterized as a function of temperature. The lowest temperature at which the toner adheres to the support medium is called the cold offset temperature: the maximum temperature at which the toner does not adhere to the fuser roll is called the hot offset temperature. When the fuser temperature exceeds a hot offset temperature, molten toner may adhere to the fuser roll during fixing, be subsequently transferred to substrates (phenomenon known as "offsetting") and result in blurred images. Between the cold and hot offset temperatures of the toner is the minimum fix temperature, the minimum temperature at which acceptable adhesion of the toner to the support medium occurs. The difference between the minimum and hot fix temperatures is called the fusing latitude.
The hot roll fixing system described above and a number of toners presently used therein exhibit several problems. First, the binder resins in the toners can require a relatively high temperature in order to be affixed to the support medium. This may result in high power consumption, low fixing speeds, and reduced fuser roll and roll bearing life. Offsetting itself can present a problem.
Toner resin which has a low fix temperature, below 200.degree. C. ("low melt toner resin"), preferably below 160.degree. C., exhibits good offset temperature performance. Toners operating at lower temperatures reduce power needs and increase component life. Low melt toners reduce volatilization of release oil, such as silicone oil, which may occur during high temperature operation and cause problems when the volatilized oil condenses on other areas of the machine. Toners with a wide fusing latitude, providing liberal requirements for oil used as a release agent and improved particle elasticity, may minimize copy quality deterioration related to toner offset. Hence, the desirability of low-fix temperature toner resins, particularly for use in hot-roll fixing electrophotographic processes, is apparent.
Contemporary artisans skilled in toner resin technology have developed resins having a lower minimum fix temperature than previously available commercial resins. These resins have a lower molecular weight than other, higher fix temperature resins. U.S. Pat. No. 3,590,000 to Palermiti et al. and U.S. Pat. No. 3,681,106 to Burns et al. disclose attempts to use polyester resins as a toner binder. Although a minimum fix temperature of polyester binder resins can be lower than resins made from other materials, such as styreneacrylic and styrene-methacrylic resins, this may lead to an undesirable lower hot offset temperature, thus resulting in a decreased offset resistance. These lower molecular weight resins generally suffer from an unacceptable borderline glass transition temperature, which negatively impacts blocking of the toner occurring during toner storage.
Contemporary polyester resins used in toners, which may have advantageous lower fix temperatures than earlier toner resins, may yield poor, unacceptable toner performances over long storage periods under extreme operating conditions because the resin has a glass transition temperature lying in a range of values which differentiate between acceptable and unacceptable transition temperatures. With these borderline transition temperature resins, in a hot-roll fixing system operating at elevated temperature, heat generated by the system during high volume cycling will cause toner stored in a toner reservoir over a period of time to agglomerate, a phenomenon known as "blocking".
Upon exposure to heat, toner particles which exhibit undesirable blocking performance will begin to associate with adjacent toner particles. If temperature exceeds an experimentally-determined threshold value, the toner particles form "blocks" of agglomerated toner and thus become unsuitable for further use as toner.
Blocking performance of a toner is directly related to the glass transition temperature of the toner resin used to prepare the toner. As the glass transition temperature of the resin decreases, the temperature at which blocking of the toner occurs also drops. Polyester resins exhibiting low glass transition temperatures, yet having desirable operating characteristics, are often undesirable for use as toner resins because of poor blocking performance.
Known polyester resins, which exhibit such low fix temperature and other desirable characteristics as discussed above, often suffer from the above-mentioned drawbacks, rendering the toner resins undesirable for use in commercial toner applications.
Skilled artisans, attempting to overcome limitations of known polyester resins, have modified the resin structures by branching, cross-linking and grafting, using conventional polymerization and condensation reactions. Such processes may result in resins having one or more desirable characteristics of known polyester resins, including offset resistance. Burns, mentioned above, discloses non-linear modification of a polymer backbone by mixing a trivalent or more polyol or polyacid with monomer to generate branching during polycondensation, resulting in a resin having improved offset resistance. However, too much branching may result in an increased minimum fix temperature, diminishing any advantage of a polymer thus modified.
Similarly, U.S. Pat. No. 4,533,614 to Fukumoto et al discloses a non-linearly modified, low-melting polyester containing: 1) an alkylsubstituted dicarboxylic acid and/or an alkyl-substituted diol; 2) a trivalent or more polycarboxylic acid and/or a trivalent or more polyol; 3) a dicarboxylic acid; and 4) an etherated diphenol. The main acid component of the polyester requires 50 mole %, preferably 60 mole %, or more of an aromatic dicarboxylic acid, its analogous anhydride, or other dicarboxylic acids to impart sufficient electrophotographic charge characteristics to a toner made from the resin. Fukimoto discloses that modified polyesters having less than the disclosed, required amount of aromatic acid do not impart sufficient charge characteristics to a toner made from the disclosed resin.
However, efforts to produce low-cost polyester resins that exhibit rheological properties required in producing highly desirable low melt toner resins, without borderline glass transition temperatures, have been unsuccessful. Prior to this invention, poor blocking performance in commercially reproduced polyester resins remained a concern.
Poor resin blocking performance may have prevented use of resins, otherwise having desirable resin characteristics. The modified linear polymers of the invention provide inexpensive, higher-performing linear base resin alternatives without undesirable borderline glass transition temperatures.
The inventive linear polymer may be cross-linked to produce densely cross-linked toner resins that may be prepared for use in toners. U.S. patent application No. 07/814,782 discloses cross-linked toner resins, prepared by a process as also disclosed in U.S. patent application No. 07/814,641, both to Mahabadi et al. The disclosures of these two patent applications are entirely incorporated herein by reference.